Appending logs by default

[tomusdrw]

I believe it's better for us to have the logger append by default (people will be able to provide logs after restart).
Old behaviour is simple to replicate:

$ echo "" > $LOG_FILE
$ parity --log-file $LOG_FILE

so I don't see a reason to have separate CLI option to choose the behavior.

Closes #3596

[coveralls]

Coverage increased (+0.006%) to 85.936% when pulling ed7dde0 on log-append into 86522c8 on master.